β relaxation and low-temperature aging in a Au-based bulk metallic glass: From elastic properties to atomic-scale structure

The slow β relaxation is understood to be a universal feature of glassy dynamics. Its presence in bulk metallicglasses (BMGs) is evidence of a broad relaxation time spectrum that extends to deep within the glassy state.Despite the breadth of research devoted to this phenomenon, its microscopic origin is still not fully understood.The low-temperature aging behavior and atomic structural rearrangements of a Au49Cu26.9Si16.3Ag5.5Pd2.3 BMGare investigated in the regime of the slow β relaxation by employing an ensemble of experimental techniques suchas high-intensity synchrotron x-ray scattering, modulated differential scanning calorimetry (MDSC), dynamicmechanical analysis (DMA), impulse excitation, and dilatometry. Evidence of a distinct slow β-relaxation regimeis seen in the form of (1) an excess wing of the DMA loss modulus beginning at ∼50 ◦C, (2) a crossover effectof elastic modulus with isothermal aging at 50 °C, and (3) a broad, nonreversing and largely irreversible sub-Tgendotherm in theMDSC results. Atomic rearrangements occurring at the onset of the measured slow β-relaxationtemperature regime were found to be confined mainly to the short-range order length scale while no significantatomic rearrangements occur on the length scale of the medium-range order. Furthermore, evidence is presentedthat suggests the crossover effect in Young’s modulus is due to the evolution of chemical short-range order. Theseresults support the emergent picture of a dynamically heterogeneous glassy structure, in which low-temperaturerelaxation occurs through atomic rearrangements confined mostly to the short-range order length scale